Transmitters sense process variables in a variety of applications such as oil and gas refineries, chemical storage tank farms, or chemical processing plants. A process variable (PV) is a sensed parameter of a process or sensed property of a product, including absolute pressure, differential pressure, temperature, flow, material level, etc. One common transmitter application uses a transmitter to sense a PV representative of a process and transmits the sensed PV to a controller over cabling. For a two wire transmitter, the cabling is a twisted two wire cable set. The transmitter and controller are electrically cabled in series forming a current loop. The transmitter receives no external power and derives all its operating power from the current loop. Typically, the transmitter regulates the magnitude of current in the current loop, as a function of the sensed PV. In one standard protocol, the current ranges between 4 and 20 mA. Three and four wire transmitters use other cabling as appropriate.
Transmitters commonly have a cylindrically shaped housing with a bulkhead separating the housing into two compartments, with each compartment capped by a threaded cover. A cylinder as used in this specification is defined a solid bounded by a given curved surface and two parallel planes. An electronics compartment houses electronics for sensing and compensating the PV, and a terminal compartment houses terminals to connect the compensated PV to the cabling. The bulkhead has an electronics feedthrough between the compartments. The terminal compartment includes an externally threaded access channel through which the cabling enters the transmitter housing to connect to the transmitter. Many transmitter housings have two threaded access channels in the terminal compartment for connecting to external hollow electrical conduit. The hollow conduit forms a passageway between the controller and the transmitter which protects the cabling inside. The cabling typically contains two conductors for a two wire transmitter. The location of the access channels varies on transmitter housings, ranging from the top to the bottom of the housing. For temperature transmitters, the top is that side of the transmitter housing opposite the mounting boss. For pressure and other types of transmitters, the top is that side of the transmitter housing opposite the process sensor location.
Although transmitters are commonly used in various rugged industrial applications, problems have arisen when a transmitter is installed in a humid or high moisture operating environment. With the exception of hermetically sealed transmitters, moisture accumulation in the terminal compartment is a common problem encountered by transmitter designs. Hermetically sealed transmitters are costly and difficult to configure or repair as the hermetic seal is typically welded shut. In non-hermetically sealed transmitter, moisture condenses within the housing, sometimes filling the housing if not drained periodically. This moisture accumulation causes electrical shorting between terminals in the terminal compartment, cross talk or growth of organic or dendritic metallic matter which degrade the transmitter's performance. A dendritic growth is caused by a metallic filament formed from metal ions transported by a liquid on an insulating surface, the filament growing under the influence of a DC voltage bias. If the filament bridges across conductors, it can create low impedance leakage paths. A related problem to the condensate accumulation is intrusion of moisture into the transmitter. An unsealed or even an improperly hermetically sealed transmitter accumulates moisture inside if subjected to directed moisture such as pressure washing or driving rain. The detrimental effects of accumulated moisture are the same as condensate accumulation. In PRIOR ART FIG. 1, a transmitter shown generally at 50, has a terminal compartment 52 from which it is difficult to drain accumulated moisture. A pair of access channel openings 54 are located at a top 56 of transmitter 50 such that any moisture entering through the access channels 54 falls to the bottom of terminal compartment 52 and is trapped. Even if transmitter 50 is rotated and mounted 90.degree. in orientation, terminal compartment 52 remains partially filled to a waterline 58 as access channels 54 do not fully drain trapped moisture. Wall structure 60 within the transmitter housing juts out from the inner surfaces of compartment 52 so that even when transmitter 50 is mounted sideways, moisture must accumulate to the level of waterline 58 before draining.
Moisture can also degrade the effectiveness of a radio frequency interference (RFI) filter or feedthroughs in a transmitter. To minimize the effects of an electrically noisy process environment, PV electronics are commonly shielded in a Faraday cage formed in the transmitter by the electronics compartment, an access cover and RFI filters on electrical signal connections. If any one element of Faraday cage is compromised, the desired isolation is rendered ineffective and degrades transmitter performance. RFI filters typically include a mechanical case having a threaded exterior for screwing the RFI filter into the bulkhead. Moisture becomes a problem for the RFI filter when moisture accumulates across the conductor and the RFI filter case. A low impedance leakage path may be created between the case of the RFI filter and the conductor, thereby compromising the electrical isolation of the conductor. Another problem for the RFI filter arises in keeping the bulkhead watertight. The RFI filter screws into the bulkhead with conductors exposed on each side of the bulkhead, creating a seal. However, the sealing insertion force or torque required to screw in the threaded RFI filters undesirably stresses the RFI filters. A stressed RFI filter may sometimes degrade the transmitter performance by not providing the desired electrical isolation, and if detected during assembly can require substantial rework in the manufacturing process.
Therefore, a transmitter is desired which promotes the draining of accumulated moisture from within the transmitter. Another characteristic desired of the transmitter is a reliable feedthrough circuit that is assembled in a manner that does not stress the components during assembly.